One transmission of the kind defined above is for example described in U.S. Pat. specification No. 4 353 444 and the corresponding French published Patent Application No. FR 2 453 335 A. It includes a hydrodynamic torque convertor, with a locking clutch enclosed within its own rotatable casing. In a modification, the clutch includes a piston which is mounted for axial sliding movement on a hub, together with a clutch plate which is fixed to the hub. Tongues or labelled are formed in the said clutch plate for coupling the piston with the hub for simultaneous rotation. The response of such a system to variations in pressure is better than that of a system using simple splined mounting. In addition, it has no torsion damper, by contrast with the arrangement described in U.S. Pat. specification No. 4 274 519. For this reason, it is possible to envisage that the sliding action of the clutch piston might be controlled in order to improve the damping of vibrations, by causing the pressure to vary within the chambers defined on either side of the piston, this variation being achieved for example by inversion of the pressures.
Nevertheless, there remains a problem when the clutch plate is integral with the above mentioned tongues. The force exerted by the tongues on the piston cannot be as regular as might be desired. In addition, there is a high risk of defects occurring in the flatness of the clutch plate. In order to try and ensure a regular force distribution, it is possible to attache the tongues individually to the clutch plate, but this results in an undesirable increase in the number of separate components.